Insulating flat roof with 7" cavity with roof tear-off - Home Energy Pros2017-08-18T03:36:25Zhttp://homeenergypros.lbl.gov/forum/topics/insulating-flat-roof-with-7-cavity-with-roof-tear-off?commentId=6069565%3AComment%3A82761&feed=yes&xn_auth=nowhoa didn't see all the comme…tag:homeenergypros.lbl.gov,2012-04-20:6069565:Comment:827652012-04-20T04:24:53.970ZChristopher Retzlerhttp://homeenergypros.lbl.gov/profile/ChristopherRetzler
<p>whoa didn't see all the comments until after i posted. sounds like you are on top of things. The OCF is an effective air barrier and will stop most all convective sources of moisture, and I don't believe you need to worry about vapor diffusion in that climate... So leaving the air gap sounds good to me. I'm a cold climate guy so I'd like a second opinion as well. Where's David Butler when you need him?</p>
<p>whoa didn't see all the comments until after i posted. sounds like you are on top of things. The OCF is an effective air barrier and will stop most all convective sources of moisture, and I don't believe you need to worry about vapor diffusion in that climate... So leaving the air gap sounds good to me. I'm a cold climate guy so I'd like a second opinion as well. Where's David Butler when you need him?</p> With open cell foam you may h…tag:homeenergypros.lbl.gov,2012-04-20:6069565:Comment:827612012-04-20T04:07:03.280ZChristopher Retzlerhttp://homeenergypros.lbl.gov/profile/ChristopherRetzler
<p>With open cell foam you may have issues with condensation on the supply ductwork. Depends on the construction and specific climate conditions. How is the foam is being installed over the soffit areas?</p>
<p>Not sure why the air gap is there, do you intend to have a radiant barrier over the foam? Rigid insulation over the new sheathing/decking with any extra insulation in the cavities would be a more typical construction for an unvented flat roof assembly. This helps eliminate thermal…</p>
<p>With open cell foam you may have issues with condensation on the supply ductwork. Depends on the construction and specific climate conditions. How is the foam is being installed over the soffit areas?</p>
<p>Not sure why the air gap is there, do you intend to have a radiant barrier over the foam? Rigid insulation over the new sheathing/decking with any extra insulation in the cavities would be a more typical construction for an unvented flat roof assembly. This helps eliminate thermal bridging and takes care of air and vapor movement (but you may not need to worry about vapor in that climate). <br/><br/>Choosing a reflective roofing material is one of the most effective tools in a hot-dry climate.<br/> </p>
<p>good article:</p>
<p><a rel="nofollow" href="http://www.buildingscience.com/documents/digests/bsd-102-understanding-attic-ventilation" target="_blank">http://www.buildingscience.com/documents/digests/bsd-102-understand...</a></p> Hopefully someone else chimes…tag:homeenergypros.lbl.gov,2012-04-20:6069565:Comment:826732012-04-20T02:54:41.633ZSean Lintow Srhttp://homeenergypros.lbl.gov/profile/sls
<p>Hopefully someone else chimes in if they know diffrent, but based on my recollection, you need the gap &amp; venting with that setup. Probably be worth checking on the GBA or BSC site... (be careful with older papers)</p>
<p>Hopefully someone else chimes in if they know diffrent, but based on my recollection, you need the gap &amp; venting with that setup. Probably be worth checking on the GBA or BSC site... (be careful with older papers)</p> NEW ROOF: Apply one layer of…tag:homeenergypros.lbl.gov,2012-04-19:6069565:Comment:825842012-04-19T19:38:08.655ZCraig Birdhttp://homeenergypros.lbl.gov/profile/CraigBird
<p>NEW ROOF: Apply one layer of Glasbase and two layers of Glasply with hot asphalt between each layer, surface with emulsion, and two coats of white roof coating</p>
<p>NEW ROOF: Apply one layer of Glasbase and two layers of Glasply with hot asphalt between each layer, surface with emulsion, and two coats of white roof coating</p> This is in Tucson. It is a 13…tag:homeenergypros.lbl.gov,2012-04-18:6069565:Comment:822842012-04-18T23:16:18.512ZCraig Birdhttp://homeenergypros.lbl.gov/profile/CraigBird
<p>This is in Tucson. It is a 1300 square foot, built 1972, slump block, flat roof home with bird blocking and 2x8 roof. I prefer to create an unvented assembly, but the primary question is since we CANNOT spray to the underside of the roof deck and instead insulate to the drywall lid, AND it leaves an air space, is it ok, to have an unvented air space against the roof deck or could that cause problems? I don't want to add an additional 1.5" of foam if I do not have to, since 5.5" OC is plenty.…</p>
<p>This is in Tucson. It is a 1300 square foot, built 1972, slump block, flat roof home with bird blocking and 2x8 roof. I prefer to create an unvented assembly, but the primary question is since we CANNOT spray to the underside of the roof deck and instead insulate to the drywall lid, AND it leaves an air space, is it ok, to have an unvented air space against the roof deck or could that cause problems? I don't want to add an additional 1.5" of foam if I do not have to, since 5.5" OC is plenty. If there is a building science reason why I can not leave that air space in an unvented assembly, I would propose filling cavity. Alternatively, is the air space is ok if I maintain the bird board vents? (or is a there a compelling reason to maintain the ventilation?)</p> Actually so I can stop guessi…tag:homeenergypros.lbl.gov,2012-04-18:6069565:Comment:822802012-04-18T18:56:28.478ZSean Lintow Srhttp://homeenergypros.lbl.gov/profile/sls
<p>Actually so I can stop guessing at what you are looking at &amp; thinking - where is this property at (city is fine &amp; maybe the style of the house, age)? Something I would have done/recommend for Phoenix could be completely different than what I might recommend in another "hot dry climate" area or even different structures. </p>
<p>Yes cavities are filled all the time, but generally it is directly against the sheathing. The vents are there for reason - now if they are still needed in…</p>
<p>Actually so I can stop guessing at what you are looking at &amp; thinking - where is this property at (city is fine &amp; maybe the style of the house, age)? Something I would have done/recommend for Phoenix could be completely different than what I might recommend in another "hot dry climate" area or even different structures. </p>
<p>Yes cavities are filled all the time, but generally it is directly against the sheathing. The vents are there for reason - now if they are still needed in your case is still up in the air. On the steep curve of foam &amp; the cost mentioned in another reply, you aren't even close to it with OC in this area </p> Not sure of your point in reg…tag:homeenergypros.lbl.gov,2012-04-18:6069565:Comment:822792012-04-18T18:10:28.292ZCraig Birdhttp://homeenergypros.lbl.gov/profile/CraigBird
<p>Not sure of your point in regards to leaving a space. Cavities are filled with foam all the time. The space in this situation is between the top of the foam product (applied to drywall) and the roof deck.</p>
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<p>I will need to contact roofer about what he is applying to the roof.</p>
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<p>Not sure of your point in regards to leaving a space. Cavities are filled with foam all the time. The space in this situation is between the top of the foam product (applied to drywall) and the roof deck.</p>
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<p>I will need to contact roofer about what he is applying to the roof.</p>
<p></p> Talking about insulating the…tag:homeenergypros.lbl.gov,2012-04-18:6069565:Comment:822782012-04-18T18:04:38.749ZCraig Birdhttp://homeenergypros.lbl.gov/profile/CraigBird
<p>Talking about insulating the roof, not coating stuff on top. The ducts are contained within the dropped ceiling and are under the roof.</p>
<p>Talking about insulating the roof, not coating stuff on top. The ducts are contained within the dropped ceiling and are under the roof.</p> We would not fill rafter comp…tag:homeenergypros.lbl.gov,2012-04-18:6069565:Comment:823912012-04-18T18:03:33.969ZCraig Birdhttp://homeenergypros.lbl.gov/profile/CraigBird
<p>We would not fill rafter completely due to cost. Foam = expensive and you hit diminishing returns on a pretty steep curve. On the dropped ceilings, we would be spraying directly against the heating ducts, drywall and dropped ceiling/wall junctures to complete an air barrier and insulate kneewalls created by drop.</p>
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<p>I wouldn't expect much ventilation at all either. I would be concerned about that heat build up in the one inch air gap beneath the sheathing, so maybe we will have…</p>
<p>We would not fill rafter completely due to cost. Foam = expensive and you hit diminishing returns on a pretty steep curve. On the dropped ceilings, we would be spraying directly against the heating ducts, drywall and dropped ceiling/wall junctures to complete an air barrier and insulate kneewalls created by drop.</p>
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<p>I wouldn't expect much ventilation at all either. I would be concerned about that heat build up in the one inch air gap beneath the sheathing, so maybe we will have to add the additional cost of filling the cavity.</p> Craig if you leave a space yo…tag:homeenergypros.lbl.gov,2012-04-18:6069565:Comment:824632012-04-18T12:08:30.338ZSean Lintow Srhttp://homeenergypros.lbl.gov/profile/sls
<p>Craig if you leave a space you need to leave some sort of opening from what I recall (think expansion &amp; contraction) - if you spray directly against the sheathing you wouldn't. Of course this is also dependent on how you are finishing the roof - is foam going on top, hot mopped, rubber, or...(If you have foam going directly on top, the cavities need to be filled)</p>
<p>Performance wise - it will probably be better than what was there before</p>
<p>Electrical &amp; mechanicals - besides…</p>
<p>Craig if you leave a space you need to leave some sort of opening from what I recall (think expansion &amp; contraction) - if you spray directly against the sheathing you wouldn't. Of course this is also dependent on how you are finishing the roof - is foam going on top, hot mopped, rubber, or...(If you have foam going directly on top, the cavities need to be filled)</p>
<p>Performance wise - it will probably be better than what was there before</p>
<p>Electrical &amp; mechanicals - besides them being encased in foam / harder to get to later should there be an issue, there really shouldn't be any issues. With that said, check the wires &amp; everything carefully for damage &amp; be mindful of large groups being bundled together.</p>
<p>David, I think he is talking about interior soffits / bulk heads which encase the ducts which is where a great number of issues generally are, as most of them just are framed with no drywall inside. I don't see an issue with foam being used around the ducts but make sure you check those areas carefully &amp; fix any duct / air sealing issues</p>
<p>Roof coating - that gets back to what your roofing material is &amp; the best product to go over it. With the sun beating down on that roof a "cool roof" coating is generally a good bet </p>